The invention relates to a method of manufacturing a display tube comprising a display screen having an upright edge and a cone, in which the display screen and the cone are subjected to temperature treatments during which measures are taken to reduce stresses occurring in the display screen.
A display tube is usually composed of a display screen and a cone which are affixed to each other by means of a glass frit provided on the edge of the cone. In a so-called adhering jig the display screen is accurately positioned on the cone. The cone and the display screen are then passed through a furnace in which the glass frit recrystallizes during heating at a fixed temperature range so that the display screen and the cone are joined rigidly and in a vacuum-tight manner.
After placing the electron gun in the neck of the display tube, the display tube is evacuated while heating. During pumping, the display tube is passed through a furnace in which the display tube is degassed while it is heated at a fixed temperature range. During the cooling process the tube is sealed in a vacuum-tight manner.
A method of the kind mentioned in the opening paragraph is disclosed in U.S. Pat. No. 4,152,036. During the temperature treatments in manufacturing the display tube, stresses occur in the display screen. Upon vacuum pumping the display tube to which the above Patent relates in particular, these stresses are caused by the temperature difference and by the pressure differential between the outside and the inside of the display tube. These stresses may cause fracture of the display tube. In order to counteract the stresses a mechanical pressure is exerted on parts of the periphery, in particular the corners of the display screen. For that purpose, a display tube is placed in a jig which has four arms which are secured approximately in their centres to the jig so as to be rotatable. At one end the arms are connected to a ring and at their other end they engage the corners of the display screen. When the ring expands the arms pivot about midpoints so that the ends exert a pressure on the corners of the display screen which is proportional to the temperature of the display tube.
However, the jig is of a complicated construction which is not well suited for use in mass production. Moreover, the jig is not suitable for use in affixing the display screen to the cone.
For a better understanding, the occurrence of stresses in the display screen during the various temperature treatments will now be described in greater detail.
When affixing the display screen to the cone a temperature difference occurs between the outside and the inside of the tube, which results in tensile stresses in the tube. As a result of the temperature difference between the inside and the outside, the display screen will dome so that the upright edge of the display screen is drawn inwardly. Due to greater rigidity, the corners of the display screen are drawn inwardly to a comparatively smaller extent than the centres of the sides of the upright edge of the display screen. This causes tensile stresses in the corners of the display screen which are added to the tensile stresses which are caused by the temperature difference between the outside and the inside of a corner of the display screen.
It has also been found that in colour display tubes in which a colour selection electrode is suspended from suspension means connected in the corners of the display screen, said suspension means cause additional tensile stresses in the corners of the display screen. These additional tensile stresses in such display tubes result in a larger percentage of fracture than in display tubes in which the colour selection electrode, as is usual nowadays, is suspended from suspension means connected in the centres of the sides of the upright edge of the display screen.
Upon vacuum pumping the display tube, the above tensile stresses also occur. These tensile stresses are larger than those occurring when affixing the display screen because the warm-up speed is faster. These larger tensile stresses are partly compensated for by the compressive stresses occurring as a result of the vacuum pumping, which stresses are directed opposite to the tensile stresses.
The tensile stresses occurring when affixing the screen and vacuum pumping the display tube cause a percentage of fractures resulting in considerable losses, in particular in colour display tubes in which the colour selection electrode is suspended in the corners of the display screen.